The EGR ratio measuring device is so arranged that a reflux flow channel (R) connecting an exhaust gas pipe (EXT) with an intake air pipe (INT) of an internal combustion engine (EG) is provided as shown in FIG. 4 so that a combustion temperature is lowered by refluxing the exhaust gas to the intake air and eventually reduces NOX. As shown in the patent document 1, an EGR ratio is known as a parameter to control the refluxing amount by an EGR valve (V) and to optimize combustion of the internal combustion engine (EG).
The EGR ratio is a ratio of the gas recirculation amount from the exhaust gas pipe (EXT) to the intake air amount including the recirculation amount. A CO2 concentration at an intake air side and a CO2 concentration at an exhaust gas side are measured and the EGR ratio can be calculated from the measured value of the CO2 concentration by the use of the following equation (1).EGR=([CO2]int−[CO2]amb)/([CO2]ext−[CO2]amb)  (1)                EGR:EGR ratio        [CO2]int:CO2 concentration at intake air side        [CO2]ext:CO2 concentration at exhaust gas side        [CO2]amb:CO2 concentration in inhaled fresh air (atmosphere)        
If [CO2]amb is ignored, the EGR ratio can be calculated by the following expression (2).EGR=[CO2]int/[CO2]ext  (2)
At this time, in order to eliminate an influence of interference due to water on the CO2 concentration, a dehumidifier is arranged in mid-course from a sample point to a CO2 analyzer so that the sample gas is dehumidified (non-patent document 1).